Effects of Superhydrophobic and Superhydrophilic Surfaces on Heat Transfer and Oscillating Motion of an Oscillating Heat Pipe

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
Tingting Hao ◽  
Xuehu Ma ◽  
Zhong Lan ◽  
Nan Li ◽  
Yuzhe Zhao
Keyword(s):  
Heat Transfer ◽  
Superhydrophobic Surface ◽  
Thin Liquid Film ◽  
Maximum Amplitude ◽  
Line Length ◽  
Copper Surface ◽  
Working Fluid ◽  
Liquid Slug ◽  
Vapor Interface ◽  
Liquid Vapor

The effects of superhydrophobic surface and superhydrophobic and superhydrophilic hybrid surface on the fluid flow and heat transfer of oscillating heat pipes (OHPs) were investigated in the paper. The inner surfaces of the OHPs were hydrophilic surface (copper), hybrid surface (superhydrophilic evaporation and superhydrophobic condensation section), and uniform superhydrophobic surface, respectively. Deionized water was used as the working fluid. Experimental results showed that superhydrophobic surface influenced the slug motion and thermal performance of OHPs. Visualization results showed that the liquid-vapor interface was concave in the OHP with copper surface. A thin liquid film existed between the vapor plug and the wall of the OHP. On the contrary, the liquid-vapor interface took a convex profile in the OHP with superhydrophobic surface and the liquid-vapor interface contact line length in the hybrid surface OHP was longer than that in the uniform superhydrophobic surface OHP. The liquid slug movements became stronger in the hybrid surface OHPs as opposed to the copper OHP, while the global heat transfer performance of the hybrid surface OHPs increased by 5–20%. Comparing with the copper OHPs, the maximum amplitude and velocity of the liquid slug movements in the hybrid surface OHPs increased by 0–127% and 0–185%, respectively. However, the maximum amplitude and velocity of the liquid slug movements in the uniform superhydrophobic OHPs was reduced by 0–100% and 0–100%, respectively. The partial dryout phenomenon took place in OHPs with uniform superhydrophobic surface. The liquid slug movements became weaker and the thermal resistance was increased by 10–35% in the superhydrophobic surface OHPs.

Heat Transfer and Pressure Drop Characteristics of Condensation for R410A in a 3.78mm Circular Tube Under Normal and Micro Gravity

2016 ◽  
Author(s):  
Jingzhi Zhang ◽  
Wei Li ◽  
Tom I.-P. Shih ◽  
Yonghai Zhang ◽  
Yanping Shi ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Liquid Film ◽  
Thin Liquid Film ◽  
Transfer Coefficients ◽  
Bottom Part ◽  
Vapor Interface ◽  
Heat Transfer Coefficients ◽  
Micro Gravity ◽  
Liquid Vapor

Heat transfer and pressure drop characteristics of condensation for R410A inside horizontal tubes (dh = 3.78 mm) under normal and micro gravity are investigated numerically. The Volume of Fluid method is used to acquire liquid-vapor interface, while the low-Reynolds form of the Shear Stress Transport k∼ω (SST k∼ω) model is adopted to taking turbulent effect into account. The results indicate that the heat transfer coefficients decrease with increasing gravity accelerations, while the frictional pressure gradients increase with increases in gravity accelerations. The liquid film accumulates at the bottom of the tube, leading to a very thin liquid film attached to the upper part of inner tube wall. This accumulation effect decreases with decreases in gravitational accelerations. A more symmetrical liquid-vapor interface is obtained at lower gravity. The average liquid film thickness is nearly the same for different gravity accelerations at the same vapor quality (δave≈56 μm at x = 0.9 and δave≈230 μm at x = 0.5). The local heat transfer coefficients increase with increasing gravity at the top of the tube and decrease with increases in gravity at the bottom, while the bottom part of the tube has a limited contribution to the global heat transfer coefficient for stratified flow regime. The numerical data obtained under normal gravity agree well with well-known empirical correlations.

Steady State Numerical Modeling of Non-Conventional Loop Heat Pipes (LHPs)

2012 ◽  
Author(s):  
Karthik S. Remella ◽  
Frank M. Gerner ◽  
Ahmed Shuja ◽  
Praveen Medis
Keyword(s):  
Heat Transfer ◽  
Steady State ◽  
Single Phase ◽  
Heat Pipes ◽  
Working Fluid ◽  
Vapor Interface ◽  
Loop Heat Pipes ◽  
Led Array ◽  
Fill Volume ◽  
Liquid Vapor

Loop heat pipes (LHPs) transport energy from an evaporator to a condenser in the form of latent heat. In conventional LHPs, the vapor pressure is significantly higher than the liquid pressure across the liquid-vapor interface due to the small pores and the corresponding capillary forces in the wick. This large pressure difference transports the single phase vapor after evaporation from the evaporator to the condenser and once the vapor is condensed, a single phase liquid from the condenser back to the evaporator. This current work involves the development of a steady state design model of the LHP system consisting of a planar evaporator package and a finned copper tube loop, which serves as an air-cooled condenser. Although evaporation due to the heat transfer creates the pressure in the vapor which drives the flow, contrasting to the conventional loop heat pipes, the pressure drop across the liquid-vapor interface is much smaller. A positive hydrostatic head is applied to the liquid above the wick and there is entrainment of liquid from the wick in the evaporator. Therefore, the fluid flow leaving the evaporator package is a two-phase flow, assumed to be saturated liquid and saturated vapor in equilibrium. The primary objective of this non-conventional LHP device is to remove the thermal energy dissipated from a Light Emitting Diode (LED) array. A major portion of this energy causes evaporation of the working fluid within the wick. The remaining energy reheats the liquid in both the liquid return line and within the evaporator package. The evaporator package is modeled as a one-dimensional thermal resistance network and these resistances are empirically determined from experiments. It is found that the convective heat transfer co-efficient of air plays a pivotal role in the heat dissipation and hence, is empirically determined in this work. This value is fairly agreeable with the Nusselt number correlation on the air side developed by Hahne et al. [1]. A mass balance relates the fill volume with the length of the condenser. The temperatures within the LHP are predicted by applying the principle of conservation of energy over the evaporator, the condenser and the sub-cooler sections of the heat exchanger loop. Finally, this LHP model predicts an approximate fill volume necessary for the LHP to operate properly.

Instantaneous Optical Measurement of the Temperature at the Interface Between a Wall and a Thin Liquid Film

2020 ◽  
Vol 142 (12) ◽  
Author(s):  
Brian E. Fehring ◽  
Roman W. Morse ◽  
Jason Chan ◽  
Kristofer M. Dressler ◽  
Evan T. Hurlburt ◽  
...  
Keyword(s):  
Laser Beam ◽  
Liquid Film ◽  
Solid Wall ◽  
Incident Angle ◽  
Thin Liquid Film ◽  
Index Of Refraction ◽  
High Temporal Resolution ◽  
Flow Measurements ◽  
Vapor Interface ◽  
Liquid Vapor

Abstract Instantaneous temperature measurements at the interface between a solid wall and a thin, unsteady liquid film are performed using thermoreflectance, a nonintrusive optical technique with high temporal resolution. A laser beam is directed at a wall–liquid interface, and the intensity of the light reflected at that interface is measured by a photodiode. The intensity of the reflected light varies with the index of refraction of the liquid at the wall. The index of refraction is a function of temperature, which enables the instantaneous measurement of the wall temperature. In the presence of thin liquid films, reflections from the liquid–vapor interface at the free surface of the film generate noise in the measurements. We demonstrate that orienting the laser beam at a large incident angle, close to total internal reflection, minimizes noise from the liquid–vapor interface while increasing the sensitivity of the measurement. The thermoreflectance technique is validated in an unsteady two-phase annular flow. Measurements of temperature fluctuations less than 1 K in amplitude are achieved, with an uncertainty of 0.1 K.

Nucleate Boiling of Dielectric Liquids on Hydrophobic Patterned Surfaces

2014 ◽  
Author(s):  
Nihal E. Joshua ◽  
Denesh K. Ajakumar ◽  
Huseyin Bostanci
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Nucleate Boiling ◽  
High Heat ◽  
Copper Surface ◽  
Dielectric Liquid ◽  
Working Fluid ◽  
Patterned Surfaces ◽  
High Heat Flux ◽  
Dielectric Liquids

This study experimentally investigated the effect of hydrophobic patterned surfaces in nucleate boiling heat transfer. A dielectric liquid, HFE-7100, was used as the working fluid in the saturated boiling tests. Dielectric liquids are known to have highly-wetting characteristics. They tend to fill surface cavities that would normally trap vapor/gas, and serve as active nucleation sites during boiling. With the lack of these vapor filled cavities, boiling of a dielectric liquid leads to high incipience superheats and accompanying temperature overshoots. Heater samples in this study were prepared by applying a thin Teflon (AF400, Dupont) coating on 1-cm2 smooth copper surfaces following common photolithography techniques. Matching size thick film resistors, attached onto the copper samples, generated heat and simulated high heat flux electronic devices. Tests investigated the heater samples featuring circular pattern sizes between 40–100 μm, and corresponding pitch sizes between 80–200 μm. Additionally, a plain, smooth copper surface was tested to obtain reference data. Based on data, hydrophobic patterned surfaces effectively eliminated the temperature overshoot at boiling incipience, and considerably improved nucleate boiling performance in terms of heat transfer coefficient and critical heat flux over the reference surface. Hydrophobic patterned surfaces therefore demonstrated a practical surface modification method for heat transfer enhancement in immersion cooling applications.

Prediction of the Transport Phenomena in the Micro Capillary Tube of a CPL System

2003 ◽  
Author(s):  
Kyoungwoo Park ◽  
Kwan-Soo Lee
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Capillary Tube ◽  
Transport Phenomena ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Two Phase ◽  
Vapor Interface ◽  
Meniscus Region ◽  
Liquid Vapor

A mathematical model is presented to predict the two-phase transport phenomena of the evaporating extended meniscus region in a micro capillary tube which approximates the evaporator of the CPL system. The behavior of a liquid-vapor interface can be estimated by using the augmented Laplace-Young equation. The governing equations for transport fields of liquid and vapor phases can be obtained by adopting the different physical approaches for the meniscus and thin film regions. In this model, the variation of vapor pressure and the disjoining pressure effect are included and the friction force at the liquid-vapor interface is also considered. The results show that the local heat transfer coefficient has an extremely large value in the thin film region. However, the amount of heat transfer rate, of the meniscus region is larger than that of the thin film region. It is also found that the length of the extended meniscus region is affected by the heat flux, the tube radius and the dispersion constant.

Inverse-thermocapillary evaporation in a thin liquid film of self-rewetting fluid

2020 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Elaine Lim ◽  
Tze Cheng Kueh ◽  
Yew Mun Hung
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Liquid Film ◽  
Temperature Region ◽  
Thin Liquid Film ◽  
Thermocapillary Flow ◽  
Working Fluid ◽  
Surface Tension Gradient ◽  
Thermocapillary Effect ◽  
Content Type

Purpose The present study aims to investigate the inverse-thermocapillary effect in an evaporating thin liquid film of self-rewetting fluid, which is a dilute aqueous solution (DAS) of long-chain alcohol. Design/methodology/approach A long-wave evolution model modified for self-rewetting fluids is used to study the inverse thermocapillary characteristics of an evaporating thin liquid film. The flow attributed to the inverse thermocapillary action is manifested through the streamline plots and the evaporative heat transfer characteristics are quantified and analyzed. Findings The thermocapillary flow induced by the negative surface tension gradient drives the liquid from a low-surface-tension (high temperature) region to a high-surface-tension (low temperature) region, retarding the liquid circulation and the evaporation strength. The positive surface tension gradients of self-rewetting fluids induce inverse-thermocapillary flow. The results of different working fluids, namely, water, heptanol and DAS of heptanol, are examined and compared. The thermocapillary characteristic of a working fluid is significantly affected by the sign of the surface tension gradient and the inverse effect is profound at a high excess temperature. The inverse thermocapillary effect significantly enhances evaporation rates. Originality/value The current investigation on the inverse thermocapillary effect in a self-rewetting evaporating thin film liquid has not been attempted previously. This study provides insights on the hydrodynamic and thermal characteristics of thermocapillary evaporation of self-rewetting liquid, which give rise to significant thermal enhancement of the microscale phase-change heat transfer devices.

Force Analysis of Bubble Dynamics in Flow Boiling Silicon Nanowire Microchannels

2016 ◽  
Author(s):  
Tamanna Alam ◽  
Wenming Li ◽  
Fanghao Yang ◽  
Ahmed Shehab Khan ◽  
Yan Tong ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Bubble Growth ◽  
Bubble Dynamics ◽  
Flow Boiling ◽  
Silicon Nanowire ◽  
Bubble Nucleation ◽  
Force Analysis ◽  
Vapor Interface ◽  
Liquid Vapor

In microchannel flow boiling, bubble nucleation, growth and flow regime development are highly influenced by channel cross-section and physical phenomena underlying this mechanism are far from being well-established. Relative effects of different forces acting on wall-liquid and liquid-vapor interface of a confined bubble play an important role in heat transfer performances. Therefore, fundamental investigations are necessary to develop enhanced microchannel heat transfer surfaces. Force analysis of vapor bubble dynamics in flow boiling Silicon Nanowire (SiNW) microchannels has been performed based on theoretical, experimental and visualization studies. The relative effects of different forces on flow regime, instability and heat transfer performances of flow boiling in Silicon Nanowire microchannels have been identified. Inertia, surface tension, shear, buoyancy, and evaporation momentum forces have significant importance at liquid-vapor interface as discussed earlier by several authors. However, no comparative study has been done for different surface properties till date. Detailed analyses of these forces including contact angle and bubble flow boiling characteristics have been conducted in this study. A comparative study between Silicon Nanowire and Plainwall microchannels has been performed based on force analysis in the flow boiling microchannels. In addition, force analysis during instantaneous bubble growth stage has been performed. Compared to Plainwall microchannels, enhanced surface rewetting and critical heat flux (CHF) are owing to higher surface tension force at liquid-vapor interface and Capillary dominance resulting from Silicon Nanowires. Whereas, low Weber number in Silicon Nanowire helps maintaining uniform and stable thin film and improves heat transfer performances. Moreover, force analysis during instantaneous bubble growth shows the dominance of surface tension at bubble nucleation and slug/transitional flow which resulted higher heat transfer contact area, lower thermal resistance and higher thin film evaporation. Whereas, inertia force is dominant at annular flow and it helps in bubble removal process and rewetting.

Visualization of the Evaporating Liquid-Vapor Interface in Micropillar Arrays

2016 ◽  
Vol 138 (2) ◽  
Author(s):  
Dion S. Antao ◽  
Solomon Adera ◽  
Edgardo Farias ◽  
Rishi Raj ◽  
Evelyn N. Wang
Keyword(s):  
Capillary Pressure ◽  
Evaporation Rate ◽  
Thin Liquid Film ◽  
Monochromatic Light ◽  
Unit Cell ◽  
Vapor Interface ◽  
Viscous Loss ◽  
Liquid Vapor ◽  
Micropillar Array ◽  
Micropillar Arrays

We captured interesting static and dynamic behavior of the liquid-vapor interface in well-defined silicon micropillar arrays during thermally driven evaporation of water from the microstructured surface. The 3-D shape of the meniscus was characterized via laser interferometry where bright and dark fringes result from the interference of incident and reflected monochromatic light due to a variable thickness thin liquid film (FIG. 1). During steady state evaporation experiments, water was supplied to the sample with a syringe pump at 10 μL/min. FIG. 2a and 2b show a SEM image of a typical fabricated micropillar array and a schematic of the experimental setup, respectively. When water wicks through the micropillar array, the meniscus in a unit cell (four pillars in FIG. 1) assumes an equilibrium shape depending on the location from the liquid source/reservoir and the ambient conditions (ambient evaporation at Qin = 0 W). At this point, the meniscus is pinned at the top of the pillars. As the evaporation rate increases due the applied heat flux, the meniscus increases in curvature, thus increasing the capillary pressure to sustain the higher evaporation rate. This is evidenced by the increasing number of fringes in the unit cell when Qin is increased (0 W, 0.11 W, 0.44 W, and 0.99 W, FIG. 1a-1d respectively). Beyond a maximum curvature, the meniscus de-pins from the pillar top surface and recedes within the unit cell. This occurs when the capillary pressure generated at this curvature, cannot balance the viscous loss resulting from flow through the micropillar array. We observed that this receding shape was independent of the applied heat, and only depended on the micropillar array geometry and the intrinsic wettability of the material. Representative meniscus profiles along the diagonal direction of the unit cell obtained from image analysis of FIG. 1 at various Qin are shown in FIG. 2c.

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